. Bulletin. Science. FEEDING ECOLOGY OF TILAPIA 29. 80% ? Spring D Summer EH! Fall m Winter Fig. 2. Mean percentages of ingested sediment over all stations by season present in tilapia stom- achs sampled at the Salton Sea. Spring n = 43, summer n = 38, fall n = 38, winter n = 33. Mean values were calculated from individual stomachs. Because of heteroskedasticity in the data, proportions were analyzed using dis- tribution-free statistical methods. Sediment and Salton Sea biota groups were an- alyzed for seasonal and spatial patterns. Kruskal-Wallis tests (Sheskin 2000) were used to assess seaso
. Bulletin. Science. FEEDING ECOLOGY OF TILAPIA 29. 80% ? Spring D Summer EH! Fall m Winter Fig. 2. Mean percentages of ingested sediment over all stations by season present in tilapia stom- achs sampled at the Salton Sea. Spring n = 43, summer n = 38, fall n = 38, winter n = 33. Mean values were calculated from individual stomachs. Because of heteroskedasticity in the data, proportions were analyzed using dis- tribution-free statistical methods. Sediment and Salton Sea biota groups were an- alyzed for seasonal and spatial patterns. Kruskal-Wallis tests (Sheskin 2000) were used to assess seasonal and spatial patterns in feeding. Macrophytes were analyzed for spatial patterns only. The average percent of zooplankton in tilapia stomachs was compared with the percentages of taxa in plankton tows using a chi-square goodness-of-fit test (Sokal and Rohlf 1995) to estimate feeding electivity. Only barnacles, copepods, and rotifers were used because of the low occurrences of other taxa. Percent occurrence from plankton tows were the estimate for the ex- pected value and the percentages from tilapia stomachs were the estimate for the observed value when conducting the chi-square test. Feeding activity was estimated from the average acidity of the upper, mid, and lower section of each stomach (Moriarty 1973). A one-way analysis of variance (ANOVA) with color of intestinal material as the factor was used to assess the effect of stomach acidity on food assimilation. Diel and seasonal feeding activity was examined with a split-plot ANOVA (Winer et al. 1991), with time of day as the within factor and season as the between factor. Results The sediment proportion in stomachs was lowest during the summer (Figure 2; Kruskal-Wallis test statistic = , p < , 3 df)- Macrophyte proportions were highest in river mouth areas (Figure 3; Kruskal-Wallis test statistic = , p = , 2 df)- Diatoms were the dominant phytoplankton in the stomachs at all seasons and sampling areas. D
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